Which one of the following statements about earthquake waves is not correct?
- (a)P waves move faster and are the first to arrive at the surface.
- (b)P waves can travel through gaseous, liquid and solid materials.
- (c)Seismographs located beyond 145° from epicentre can record the arrival of P waves.
- (d)P waves have maximum area covered under its shadow zone.
Correct — D, the claim that P waves have the maximum shadow-zone area. That is the statement which is not correct, and the question asks for exactly that. The P-wave shadow zone is a belt lying between about 105 degrees and 145 degrees from the epicentre; beyond 145 degrees, P waves that have been refracted through the earth's core reappear on seismographs. The S-wave shadow zone is far larger, because S waves are transverse and cannot pass through the liquid outer core at all — they are absent from the entire region beyond about 105 degrees, a little over 40 per cent of the earth's surface. The maximum shadow area therefore belongs to S waves, not to P waves.
- (a)P waves move faster and are the first to arrive at the surface. — This statement is correct, so it cannot be the answer to a question asking for the incorrect one. The letter P stands for primary precisely because these are the fastest seismic waves and register first on a seismograph.
- (b)P waves can travel through gaseous, liquid and solid materials. — Also correct. P waves are longitudinal compressional waves, and compression can be transmitted by gases, liquids and solids alike — which is why P waves cross the liquid outer core while S waves do not.
- (c)Seismographs located beyond 145° from epicentre can record the arrival of P waves. — Also correct. The P-wave shadow zone ends at about 145 degrees; past that angle, P waves refracted through the core emerge and are recorded, though S waves are still missing.
Earthquakes generate body waves that travel through the earth's interior and surface waves that travel along it. The two body waves are P waves, which are longitudinal and compress and stretch the material along the direction of travel, and S waves, which are transverse and shear the material at right angles to the direction of travel. Because shear cannot be sustained in a fluid, S waves stop dead at the liquid outer core, while P waves pass through it but are sharply refracted. The pattern of where each wave is and is not recorded is the primary evidence for the earth's layered interior.
Two numbers settle this whole family of questions. Both P and S waves are recorded everywhere within about 105 degrees of the epicentre. Beyond that, S waves vanish entirely — their shadow zone covers a little over 40 per cent of the earth's surface. P waves are missing only in the belt between about 105 and 145 degrees and reappear beyond 145 degrees. So the S-wave shadow zone is the larger one, and option (d) inverts that. Because the stem asks for the statement that is not correct, the inverted claim is the answer.
- P waves are longitudinal and travel through solids, liquids and gases; S waves are transverse and travel only through solids.
- P waves are the fastest seismic waves and are recorded first on a seismograph, which is why they are called primary waves.
- The P-wave shadow zone is a belt between about 105 degrees and 145 degrees from the epicentre.
- The S-wave shadow zone extends beyond about 105 degrees and covers a little over 40 per cent of the earth's surface, making it much larger than the P-wave shadow zone.
The S-wave shadow zone is the larger one, so the claim that P waves have the maximum shadow area is the incorrect statement — option (d).
- Missing the word 'not' in the stem and marking one of the three true statements.
- Swapping the two shadow zones — the S-wave shadow zone is the bigger one, and the 105-to-145-degree belt is the P-wave one.
A 'which statement is NOT correct' item built around the 105-degree and 145-degree shadow-zone boundaries, or a two-statement set on the same numbers.
Consider the following statements: 1. In a seismograph, P waves are recorded earlier than S waves. 2. In P waves, the individual particles vibrate to and fro in the direction of wave propagation whereas in S waves, the particles vibrate up and down at right angles to the direction of wave propagation. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(c) Both 1 and 2
The same two body waves, tested on speed and particle motion. It confirms option (a) of this NDA question — P waves are recorded first — and supplies the reason S waves cannot cross a liquid, which is what makes their shadow zone the larger one.
Which of the following statements with reference to shadow zones in an event of an earthquake is/are correct? 1. Zone between 105° and 145° from epicentre was identified as the shadow zone for both P-waves and S-waves 2. The shadow zone of P-waves is much larger than that of the S-waves Select the answer using the code given below :
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(a) 1 only
The identical misconception, planted three years later. Statement 2 there is the same false claim as option (d) here — that the P-wave shadow zone is the bigger one. NDA clearly rates this as a reliable trap.
Which of the following statements regarding earthquakes is/are correct? 1. The point on the Earth’s surface directly above the focus is called the epicentre of the earthquake. 2. Earthquakes generate Primary and Secondary waves that radiate outward from the earthquake focus. 3. Deep-focus earthquakes are likely to cause more damage than shallow-focus earthquakes. Select the answer using the code given below:
- (a) 1 only
- (b) 1 and 2 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer(b) 1 and 2 only
The neighbouring vocabulary — focus, epicentre and the radiation of primary and secondary waves — which the 105-degree and 145-degree shadow-zone measurements are taken from.
- practice — not a real PYQ
The shadow zone of P-waves lies between which angular distances from the epicentre?
- (a)0° and 105°
- (b)105° and 145°
- (c)145° and 180°
- (d)90° and 105°
Answer(b) 105° and 145° — beyond 145° refracted P waves are recorded again.
- practice — not a real PYQ
S-waves are not transmitted through the earth's outer core because the outer core
- (a)is at a very high temperature
- (b)is liquid and cannot sustain shear
- (c)is extremely dense
- (d)is composed mainly of iron and nickel
Answer(b) is liquid and cannot sustain shear — transverse waves need a medium with rigidity.